As an electricity storage device, there are known a secondary battery such as a nickel hydrogen battery, a nickel-cadmium battery, a lead storage battery, a lithium-ion secondary battery and the like and a capacitor such as an electric double layer capacitor, a lithium-ion capacitor, and the like. Of these, the lithium-ion secondary battery is used for an electric vehicle, a hybrid car, and the like. Further, the electric double layer capacitor is used as a back-up power supply in instantaneous power failure and the like.
The lithium-ion secondary battery comprises at least a positive electrode plate and a negative electrode plate. The positive electrode plate is configured by forming a positive electrode active material layer on a current collector such as an aluminum foil and the like. In the positive electrode active material layer, a transition metal oxide containing lithium, or the like is used as a positive electrode active material. The negative electrode plate is configured by forming a negative electrode active material layer on a negative electrode current collector such as a copper foil and the like. In the negative electrode active material layer, a carbon material such as graphite is used as a negative electrode active material. As a terminal for taking out current from the positive electrode plate or the negative electrode plate, a metal tab lead is used, in which the metal tab lead is welded to each of the positive electrode plate and the negative electrode plate. The welding of the metal tab lead is carried out in a portion where the current collector is exposed.
The electric double layer capacitor comprises at least a pair of electrode plates. The electrode plate is configured by forming an active material layer on a current collector such as an aluminum foil and the like. In the active material layer, a carbon material such as activated carbon and the like having large specific surface area is used as an active material. As terminal for taking out current from the electrode plates, a metal tab lead is employed in which the metal tab lead is welded to each of the electrode plates. The welding of the metal tab lead is carried out in a portion where the current collector is exposed.
In the electricity storage device, high capacity and high speed charging and discharging are being demanded to respond to applications such as an electric vehicle, electrically-powered equipment, and the like. As one measure for responding to this demand, it has been proposed that an undercoat layer is disposed between an active material layer and a current collector to reduce a resistance of a contact interface between the active material layer and the current collector (for example, Patent Documents 1 to 4). Also in an electrode plate provided with an undercoat layer, a metal tab lead is welded in a portion where a current collector is exposed, i.e., in a portion where neither the undercoat layer nor an active material layer is formed (for example, Patent Document 1).